Organic electroluminescence (“electroluminescence” will be occasionally referred to as “EL”, hereinafter) devices which utilize organic substances are expected to be useful for application as an inexpensive full color display device of the solid light emission type having a great size and various developments on the organic EL devices are being conducted. In general, an organic EL device has a construction comprising a light emitting layer and a pair of electrodes sandwiching the light emitting layer. The light emission of the organic EL device is a phenomenon in which, when an electric field is applied between the two electrodes, electrons are injected from the cathode side and holes are injected from the anode side, the electrons are recombined with the holes in the light emitting layer to form an excited state, and energy generated when the excited state returns to the ground state is emitted as light.
As compared with an inorganic light emitting diode, conventional organic EL devices requires high driving voltage and only exhibited low luminance or low efficiency of light emission. Moreover, characteristic degradation of the conventional organic EL devices was also extravagant and as a result, they were not practically used. Although recent organic EL devices are improved step by steps, it has been still demanded to develop organic EL devices operable at low driving voltage, with excellent luminance and favorable efficiency of light emission.
For example, there is disclosed such a technology using a single monoanthracene compound as an organic light emitting material (refer to Japanese Unexamined Patent Application Laid-Open No. Hei 11-3782). However, in this technology, a luminance obtained by using the material is as low as 1650 cd/m2, for example, at a current density of 165 mA/cm2, and an efficiency of light emission thereof is very low, i.e., only 1 cd/A, which is practically unusable. Also, there is disclosed a technology using a single bisanthracene compound as an organic light emitting material (refer to Japanese Unexamined Patent Application Laid-Open No. Hei 8-012600). However, in this technology, an efficiency of light emission obtained by using the material is also as low as about 1 to 3 cd/A. Therefore, further improvement of the technology has been demanded for rendering it practically usable. Further, there is disclosed a technology using a mono- or bis-anthracene compound together with a distearyl compound in an organic light emitting medium layer (refer to International Application Published under PCT No. WO 00/06402). However, the device described therein fails to show a sufficiently long half lifetime and, therefore; further improvement has been demanded.
Moreover, an aromatic diamine compound is used as a charge transporting material for an electronic photographic photosensitive article or a material for the organic EL device. Particularly regarding with the organic EL device, the aromatic diamine compound is actively developed in late years in order to use as a material for a hole injecting layer, a hole transporting layer or a light emitting layer.
When it is used as an organic EL material, because any material except having high glass transition temperature fails to exhibit heat resistance in the organic EL device, the aromatic diamine derivative having many aromatic rings such as benzene rings or heterocycles in their molecules are developed extensively.
On the other hand, a pyrene derivative being a polycyclic aromatic ring is practical as a light emitting material (refer to, for example, Japanese Unexamined Patent Application Laid-Open Nos. Hei 4-68076, 2002-63988 and 2002-329578) however, because a pyrene skeleton is highly planar and accordingly has high crystallinity, crystallization easily advances in its amorphous film condition or when it drives the device. Damages in the thin film induced by crystallization invite either degradation of luminance or non-light emitting state both of the device. Further, when it is highly planar, a molecular association easily generates, and a thin film-formation of itself causes an extension of wavelength as compared with fluorescent spectrum in monomolecular state.
In order for overcoming the above problems, it is necessary to suppress crystallization or generation of an excimer either by three-dimensionally bulky property of the pyrene derivative itself or by introducing a bulky substituent having steric repulsion with pyrene skeleton or other substituent. Besides, an introduction of a substituent different from each other will suppress crystallization further more.
For example, Japanese Unexamined Patent Application Laid-Open No. Hei 4-175395 discloses binary substituted diaminopyrenes such as 1,6-substituted product, 1,8-substituted product, etc.; and Japanese Unexamined Patent Application Laid-Open Nos. Hei 07-101911 and Hei 07-249490 disclose monoaminopyrene derivatives. Further, Japanese Unexamined Patent Application Laid-Open No. Hei 10-88122 discloses 1,3,6,8-tetra aminopyrene derivative.
However, because the binary substituted derivative easily causes molecular association and because the tetra-substituted derivative has the same substituent, they do not suppress crystallization enough.
Furthermore, although fabrication of an organic EL device employing the diaminopyrene derivative described in Japanese Unexamined Patent Application Laid-Open No. Hei 4-175395 as a doping material enables to obtain an organic EL device having an enhanced efficiency of light emission, its lifetime is not long enough, and accordingly, further improvement was demanded. Moreover, although Japanese Unexamined Patent Application Laid-Open No. Hei 10-251633 discloses 1,6-substituted diaminopyrene compound as its embodiments, an employment of the compound for a light emitting material easily cause decomposition in an occasion of vapor deposition because its molecular weight is large.